While much of what you say is true, there's still one thing at which Mikoto is a whole lot more mature than Index: Mikoto has her own life, future goals, ambitions, etc., while Index seems to have nothing but the desire to indefinitely leech off Touma. She does absolutely nothing but eat his food and try to get involved in his stuff - although much of that stuff is directly a result of her own circumstances.

Touma being as hopeless a hero of justice as Emiya Shirou even a rubber boot would understand he will never push Index away to an uncertain future even though that's exactly what should be done. Right now Index is still ever bit as much a prisoner of her past as she was while still under the direct control of Necessarius. One could view this as a vacation for her after everything that happened, but has anything suggested she actually thinks that way and is planning to somehow become an independent adult with her own future?